U.S. Pat. No. 5,038,965 discloses a finger actuated pump dispenser for pharmaceutical applications which not only discharges fluid at a predetermined pressure but also delivers a predetermined dosage regardless of the method of actuation employed. However, when the dispenser is actuated after it has been stored unused for some period, fluid will have evaporated from the volume within the actuator and the fluid pathway between the chamber seal and the finger controlled actuator. Consequently, the dose delivered to the user by the first actuation will be somewhat less than delivered by subsequent actuations. In some pharmaceutical applications, it is essential for the dispenser to deliver an accurate dose upon such first actuation.
Copending application Ser. No. 07/653,048 is directed toward a pump dispenser which eliminates this evaporation of fluid from the volume within the fluid pathway between the chamber by moving the exit chamber seal to the top of the pump. However, in this application, the inner stem and the main piston never move relative to each other. The seal is initially closed with maximum engagement of the areas of the parts forming the seal. During actuation of the dispenser, the pressure buildup within the pump chamber causes the stem and piston to move upward together relative to the outer stem and the seal is eventually opened. The areas of engagement of the parts forming the seal are gradually reduced as the actuation progresses prior to opening of the seal. Consequently, the parts forming the seal must be manufactured to very close tolerances to prevent leaking as the areas of engagement are decreased.
The present invention is directed toward a pump dispenser having a top disposed exit chamber seal and which is constructed in such manner that wider tolerances in manufacture can be tolerated, while at the same time, the top seal will not leak during actuation.